Preheat the oven to 450F convection or 500F. Line a baking sheet with parchment paper. Drizzle the parchment with 2 tablespoons olive oil and use a brush or your fingers to spread it around.
In a large bowl, use a box grater to grate the onion into the bowl (or finely chop it and add to the bowl). Add the parsley, oats, egg, salt, pepper, allspice, and nutmeg and use a fork to whisk up the egg and mix everything together. Add the meat and use your hands to mix until well combined.
Form into small ping-pong sized meatballs (I ended up with 28 meatballs) and place them on the baking sheet. Roll them around in the oil to coat. Bake for 10-12 minutes or until deeply browned on the bottom and cooked through.
Meanwhile, in a large skillet, warm the oil and/or butter over medium-high heat. Add the mushrooms, spread them out, and cook undisturbed for 3-5 minutes or until browned. Sprinkle with ½ teaspoon salt and stir to combine. Pour in the water and soy sauce and simmer rapidly for 5 minutes over medium or medium-low heat.
Pour the mushrooms/broth into a high-speed blender and blend until smooth, making sure to allow the steam to escape while blending. Add the sour cream and blend again until just combined. Pour back into the skillet, then add the meatballs. Simmer gently over low or medium-low heat for 1-2 minutes to warm everything up again, stirring occasionally. If the gravy is looking too thick (it can thicken as it sits), add a generous splash (2-4 tbsp) of water to the skillet and shake the skillet/stir gently to mix it in.
Serve the meatballs and “gravy” over your choice of base and top with a dollop of sour cream (if you’d like), plenty of fresh parsley, and black pepper.